The surdo is the the deep bassy heartbeat of samba. They mark the time for the bateria and dancers. The first samba drum you hear as a bateria approaches from a distance is the surdo. These are the biggest drums of the ensemble.
Three surdo sizes
Most groups use 3 sizes of surdos. The "primera" or first surdo is the biggest with the lowest pitch. The "segunda" or second surdo, is the middle sized with the middle pitch. And the smallest is the "terceira" or third surdo with the highest pitch.
The segunda is also called the "resposta" or response. The segunda responds to the primera.
Primera and segunda keeping time
Primeras and segundas mark the time. Most N. Americans count samba music in 4/4. So, with batucada styles we can think of the primeras as playing on beats 2 and 4. And we can think of segundas as playing on beats 1 and 3.
The terceira plays syncopated and some improvised rhythms over the top with emphasis on beats 2 and 4. These rhythms along with the caixa rhythms can be specific to each samba school. It's pretty cool to be able to tell which bateria is playing just by hearing the caixa pattern and terceira!
Mocidade Independente de Padre Miguel and São Clemente are notable exceptions to this, they have these reversed their primeras and segundas. Primeras are playing on 1 and 3, segundas are playing on 2 and 4.
Mangueira throws all of this out the window. They only use 2 surdos. The lower pitched surdos play on 2 and 4. And the higher pitched surdo mor, plays a 2 bar phrase.
For music from Bahia the primeras can play on 2 and 4 or 1 and 3. And segunda will, of course play opposite. Some people will swear that in Bahia they will only play primeras on 1 and 3, but just listen to some Ilê Aiyê CDs and you'll quickly hear that this is not always true.
In batucada, Rio style samba, surdo mallets have a hard bead head, covered in felt. The hard bead adds a lot of attack to the sound but the felt keeps it from sounding "slappy".
In Bahia, samba reggae and samba afro styles the mallet heads are a little softer. Sounding more like a deep thud.
In Rio shoulder straps are all the rage! The drum then hangs off the body at an angle. Which makes it really handy to wallop the snot out of it with your strong hand.
In Bahian styles they prefer the waist strap and the drum hangs more straight. Often the drum is slung low and hangs between the knees.
This has been your very quick overview of the surdo's role in samba. Many newbies to samba mistakenly think the surdo is an easy and boring drum to play. But the surdo carries so much responsibility to the group, and can have so much control over the group that higher level groups only allow experienced veterans to play surdos.
The surdos will drive the energy and personality of the group to a huge extent. If the surdo players are unsure or not mentally present this reflected in the sound of the entire group and comes off as lame and boring.
Playing surdo well, is a meditation.
This music is alive and constantly changing. Just because a "fact" is stated here, or you hear an expert state a "fact", does not mean that it is true everywhere for every circumstance. All living and healthy art forms, like all living and healthy people tend to breathe, grow, and change.